[tex-k] inaccuracy on page 215 of The TeXbook

[胡亚捷 (Hu Yajie)]

Page A215, line 26 says that expansion is suppressed
    Just after a $_3 token that begins math mode, to see if
    another $_3 follows.
However, any character whose category code is 3 would do, not
just $_3. For example, the following program
    \catcode`\!=3
    \catcode`\?=13 \def?{!}
    !?math!?
    \bye
typesets the equivalent of $ $math$ $ instead of $$math$$.

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Previous reports:
https://github.com/dine2014/texbook-mfbook-bugs/blob/master/bugs.md

(I don't know if I can go through every page before Nov 1...)